1. Field of the Invention
The present invention relates to a seat back framework of vehicle seat so designed to protect a body portion of seat occupant on the seat against damages in the case of a rear-end collision. In particular, the invention is directed to a framework of seat back of vehicle seat which is effective for preventing undesired upward displacement of seat occupant's back portion to a point close to an upper frame member of the seat back framework in order to avoid unpleasant pressed touch to the seat occupant's back portion or protect his or her vertebrae against damage.
2. Description of Prior Art
Hitherto, there have been known various kinds of vehicle seats designed to protect a body of seat occupant on the seat against damage in the case of a rear-end collision.
In particular, to protect the vertebrae of seat occupant in the rear-end collision case, various kinds of arrangements are known and incorporated in the vehicle seats.
For example, the U.S. Pat. No. 7,758,114 teaches a seat back framework in combination with a movable headrest, which comprises: a pressure receiving member movably provided in a seat back frame at a point where a seat occupant's back portion is to be positioned; and a movable headrest operatively connected with the pressure receiving member. The pressure receiving member itself is formed in a resiliently deformable fashion and movable in the backward and upward directions. According thereto, when a rear-end collision occurs, the upper body portion of seat occupant is abruptly moved under inertia to the seat back, but softly received by the pressure receiving member which is in turn automatically moved in backward and upward directions. Therefore, the seat occupant's back portion rested on that pressure receiving member as well as the seat occupant's head are simultaneously displaced in the likewise backward and upward directions. With such arrangement, no great impact is imparted to the back portion of seat occupant, and thus the vertebrae of the seat occupant can be protected against damage.
On the other hand, known is a protection arrangement in seat back, which does not require the afore-stated pressure receiving member, but simply requires a disconnection of support members in the seat back. In this prior art, upon occurrence of rear-end collision, support members for supporting a back portion of seat occupant are deformed or disconnected from each other by a great load applied thereto, thereby preventing a great impact from being imparted to the back portion of seat occupant. For example, as disclosed from the Japanese Laid-Open Patent Publication No. 2009-107571 or JP 2009-107571 A, a spring support assembly of substantially flat configuration is disposed in a seat back frame, such that two lateral ends of the spring support assembly are releasably attached to the respective two lateral frame members of the seat back frame by means of releasable connecting members. According thereto, normally, the seat occupant' back portion is resiliently supported by that spring support assembly, but, when a rear-end collision occurs, upon receiving an excessive degree of backward load from the seat occupant, the connecting members are forcibly deformed and disconnected from the lateral frame members of seat back frame. By virtue of such disconnection of connecting members, the spring support assembly on which the seat occupant's back portion is rested is further displaced backwardly, which prevents a great impact from being imparted to the back portion of the seat occupant and therefore his or her vertebra can be protected against damage.
Incidentally, most of ordinary seat back frameworks, inclusive of the above prior-art seat back frameworks, is formed in a rectangular shape having an upper horizontal frame member, a pair of vertical lateral frame members and a lower horizontal frame member, with some resilient support members, such as zigzag spring members, being extended among and connected with those three frame members. Further, a foam padding is securely placed on that resilient support members. With such cushiony arrangement of seat back, a back portion of seat occupant is resiliently supported with a comfortable touch and protected against impact caused by normal vibrations and lurches during running of car or vehicle.
However, the foregoing prior-art seat backs have been with the problem that, when a rear-end collision occurs, an excessive great backward load is applied from the seat occupant under inertia to the seat back, at which moment, the back portion of the seat occupant is sunk deeply into the foam padding of the seat back in backward and upward directions, while being further displaced in the backward and upward directions as the resilient support members mentioned above are correspondingly warped. In other words, due to the smooth resiliency of seat back, without any hard obstacle therein, the vertebrae of the seat occupant is rapidly displaced backwardly and upwardly, and further such displacement is accelerated, with the result that the seat occupant's vertebrae is suddenly and abruptly brought to a point quite close to the upper horizontal frame member of seat back framework, which will give an unexpected great pressure to the seat occupant's back portion. This will make the seat occupant feel an unpleasant pressed touch on the seat back or may result in a damage of the vertebra of seat occupant.